The present invention relates to ethylene copolymer compositions that are useful for fabricating films for the packaging of red fresh meat and to a process for packaging red fresh meat.
More particularly, the ethylene copolymer compositions of the present invention are based on a copolymer of ethylene with carboxylic acid and partially neutralised with ions and thus are able to react with the surface of the red fresh meat under specific processing conditions and prevent blood or water contained in the meat to accumulate between meat and packaging film.
Ethylene copolymer compositions used in the present invention are well known in the packaging industry for the packaging of cooked and fresh meat (Dupont Publication H56747 1/96). Examples of such uses are as sealants or shrink film in mono- or multilayer films for food packaging or even, more specifically, as a sealant for secondary sealing to prevent meat juice from sweeping in between the two film layers, thus avoiding micro channel leakers. They are known for their alledged superior adhesion to cooked meat and are used in cooked ham and cooked Turkey packaging.
However, this process and packaging films do not fully prevent the meat juice of leaving the meat once the package is opened or damaged due to insufficient adhesion to the fresh meat.
The ethylene copolymer composition and the process of the present invention overcomes the aforementioned problems, and yields packages that effectively increase the retention of meat juice within packaged fresh meat.
The present invention provides a flexible packaging film with a sealant layer made up of a polymer composition comprising a copolymer of ethylene and between 12 and 30, preferably between 15 and 20 wt % of xcex1,xcex2-unsaturated C3-C8 carboxylic acid, which acid is partially neutralised from 10-99.5%, preferably between 15 and 70% with at least one ion, preferably chosen from Zn, Mg and Ca, whereof Zn is he most preferred. The copolymer may further contain an acrylate.
The present invention provides a package that uses this film for the packaging of red fresh meat which is heated with the red fresh meat for seconds above the Vicat temperature, closes the cells of the red meat without changing its color and thus reduces the loss of meat juice or blood of the meat significantly compared to other sealant materials.
This invention consist of the use of ionomers with high acid comonomer content and neutralised with specific metal ions as sealant layers for multilayer films for fresh meat packaging with improved meat juice or purge retention and a process to archieve this. It has been found that for this use a copolymer of ethylene and an xcex1,xcex2-unsaturated C3-C8 carboxylic acid, preferably in an amount of 1-30% by weight, preferably acrylic acid or methacrylic acid, which carboxylic acid is partly neutralized with at least one metal ion, is most suitable. The copolymer may further contain an acrylate, and may be processed by e.g. extruding or casting a sheet or blowing a film.
The ionomer is described below:
Acid Copolymer and Ionomer
The acid copolymers an ionomers of the present invention are derived from direct copolymers of ethylene and xcex1,xcex2-ethylenically-unsaturated C3-C8 carboxylic acid (xe2x80x9cethylene-acid copolymersxe2x80x9d) by neutralization with metal ions.
By xe2x80x9cdirect copolymerxe2x80x9d, it is meant that the copolymer is made by polymerization of monomers together at the same time, as distinct from a xe2x80x9cgraft copolymerxe2x80x9d where a monomer is attached or polymerized onto an existing polymer chain. Methods of preparing such ionomers are well known and are described in U.S. Pat. No. 3,264,272 which is herein incorporated by reference. Preparation of the direct ethylene-acid copolymers on which the ionomers are based is described in U.S. Pat. No. 4.351,931 which is also incorporated by reference herein.
The ethylene-acid copolymers used to make the ionomeric copolymer of this invention can be E/X/Y copolymers where E is ethylene; X is a softening comonomer and Y is the xcex1,xcex2-ethylenically-unsaturated C3-C8 carboxylic acid, particularly acrylic or methacrylic acid. Preferably, however, the ethylene-acid copolymer is a dipolymer (no softening comonomer). The preferred acid moieties are methacrylic acid and acrylic acid.
By xe2x80x9csofteningxe2x80x9d, it is meant that the polymer is made less crystalline. Suitable xe2x80x9csofteningxe2x80x9d comonomers (X) are monomers selected from alkyl acrylate, and alkyl methacrylate, wherein the alkyl groups have from 1-12 carbon atoms which, when present, may be up to 30 (preferably up to 15, most preferably up to 5) wt % of the ethylene-acid copolymer.
A wide range of percent acid moiety in the ethylene-acid copolymer may be used. The acid moiety may be present in a range of about 1 to 30 weight percent of the acid copolymer, preferably in a range of about 12 to 25, alternatively about 15 to about 20. The ethylene-acid copolymers with high levels of acid are difficult to prepare in continuous polymerizers because of monomer-polymer phase separation. This difficulty can be avoided however by use of xe2x80x9ccosolvent technologyxe2x80x9d as described in U.S. Pat. No. 5.028.674 which is also incorporated herein by reference or by employing somewhat higher pressures than those at which copolymers with lower acid can be prepared.
The copolymer can be from about 10 to 99.5% neutralized with metal ions selected from groups Ia, Ib, IIa, IIIa, IVa, VIb and VIII of the Periodic Table of Elements such as sodium, potassium, zinc, calcium, magnesium, lithium, aluminum, nickel and chrominum. Preferred for best meat adhesion are ions such as zinc, calcium or magnesium selected from groups IIa and IVa of the periodic table, most preferred is zinc. Such neutralized ethylene acid copolymers are known in the art as xe2x80x9cionomersxe2x80x9d. Typically, neutralization will be from 10-70%. Preferably the copolymer has from about 35 to 70% of the carboxylic acid groups ionized by neutralization with metal ions.
Preferred ethylene-acid dipolymers are ethylene/acrylic acid and ethylene/methacrylic acid. Specific other copolymers include ethylene/n-butyl acrylate/acrylic acid, ethylene/n-butyl acrylate/methacrylic acid, ethylene/iso-butyl, acrylate/methacrylic acid, ethylene/iso-butyl acrylate/acrylic acid, ethylene/n-butyl methacrylate/methacrylic acid, ethylene/methyl methacrylate/acrylic acid.
Suitable ionomers for use in the present invention are commercially available under the trademark Surlyn(copyright), suitable ethylene acid copolymers are available under the trademark Nucrel(copyright), both from the DuPont Company.
Films
For the present invention the aforementioned polymer is used as the sealant layer in a multilayer structure with at least 3 layers of the type X/tie/sealant, whereas X can represent one or more structural and barrier layers with and corresponding tie layers.
The sealant layer will be in contact with the red fresh meat. The structure can be made by the blown film or the cast sheet process or by laminating films together.
Processing
During the packaging process, after the sealing of the package is complete, the entire package is heated at temperatures between the Vicat temperature and the melting temperature of the sealant material. This process is called xe2x80x9csecondary sealingxe2x80x9d and is well known in the industry. In the case of the aforementioned ionomers this will be between 50-85xc2x0 C.
The heating time has to be limited between 0.5-5 seconds.
The secondary sealing operation will provide an intense film-to-film contact there where no meat is in between the 2 films and provide additional protection against leakers and vacuum. This is particularly important if the package is damaged during distribution. In addition secondary sealed packs with ionomers described in this invention prevent migration of the meat juices within the package, minimising bacteria growth and leading to a more esthetical package.
However, upon opening of the package and releasing the vacuum the meat juice and blood is allowed to leave the red fresh meat, which results in a weight loss of the final meat.
Ionomers commonly used in this application have low acid content (between 9 and 12% by weight and are neutralized with Na and Zn.
It has been found surprisingly that by using ionomers wish elevated acid and Zn ion content and applying secondary seal temperatures above the Vicat temperature of these ionomers but below the melting temperature ionomers for short time will have the following effects:
The pores of the meat will be closed and the meat juice retained
The purge upon opening the package is reduced by 30-100%
Nearly no meat juice is present between meat and interior of the packaging film
This effect cannot be observed by the ionomers currently used for packaging meat as the acid content of these ionomers is too low to give sufficient meat adhesion and as the melting point of these ionomer is too high to obtain a good reaction with the meat that leads to closing of the pores but at the same time is low enough to prevent a colour change of the meat during the secondary heat treatment.